1. Field of the invention
The present invention relates to sensors, and, in particular, to position sensors for providing signals indicative of the position of a moving member relative to some fixed datum. By appropriate processing of the position signals, further signals indicative of the speed of movement of the moving member relative to this datum may also be derived. Sensors of this kind are particularly suitable for use with electrical machines, such as, inter alia, brushless d.c. drives and synchronous motors. The invention is particularly directed to a cylindrical embodiment of sensor especially suited to establishing commutation signals for a brushless d.c. drive.
2. Position of the prior art
Position and speed sensors of the foregoing kind form the subject of co-pending patent application Ser. No. 789,147, now U.S. Pat. No. 4,737,698. According to this application, there is provided a sensor comprising a drive or exciting winding for establishing a forward or drive field, at least one secondary or sense winding in which a voltage may be induced in the presence of said forward or drive field, and at least one conductive screen within which eddy currents are generated in the presence of said forward or drive field to establish a counter-field opposing said forward or drive field, said at least one conductive screen and said at least one secondary or sense winding being displaceable relative to one another within said forward or drive field so that said at least one secondary or sense winding may be shaded by said at least one conductive screen to a varying extent to thereby vary the voltage induced in said at least one secondary or sense winding, the relative displacement of said at least one conductive screen and said at least one secondary or sense winding being limited to substantially a single direction of displacement, said drive or exciting winding being configured so that in the absence of said at least one conductive screen said forward or drive field is substantially even over said at least one secondary or sense winding in said single direction of relative displacement of said at least one conductive screen and said at least one secondary or sense winding, said at least one secondary or sense winding being a two-terminal winding, and said at least one conductive screen having a surface region which is substantially parallel to a notional surface defined by said at least one secondary or sense winding.
In a particular configuration, said surface region of said at least one conductive screen is a sector of a cylindrical surface and said at least one secondary or sense winding is disposed about the periphery of a notional cylinder substantially concentric with said cylindrical sector surface region, said at least one conductive screen and said at least one secondary or sense winding being relatively displaceable in a circumferential direction about the substantially common axis of said cylindrical surface and said notional cylinder. In such constructions of sensor, the drive field is typically established by a pair of axially spaced apart cylindrical or solenoidal coils, each of which may be of relatively short axial extent.
The sensor of the invention of said co-pending application may also comprise a further secondary or sense winding, the progressive shading of which during said relative displacement of said at least one conductive screen and said at least one secondary or sense winding proceeds in a manner substantially identical with the progressive shading of said at least one secondary or sense winding during said relative displacement, said further secondary or sense winding being disposed relative to said at least one secondary or sense winding so that the progressive shading of said further secondary or sense winding during said relative displacement is spaced apart from that of said at least one secondary or sense winding. Preferably, the voltage variations in each said secondary or sense winding during said progressive shading follow a cyclic pattern and said further secondary or sense winding is disposed relative to said at least one secondary or sense winding so that said voltage variations are in phase quadrature.
In an especially favoured arrangement, said at least one secondary or sense winding has first and second coils, said coils being connected so that in the presence of said forward or drive field the voltage induced in the second coil in the absence of said at least one conductive screen is substantially equal in magnitude but opposite in polarity to the voltage induced in the first coil. In said especially favoured arrangement, said at least one secondary or sense winding has a plurality of coils extending in said single direction of relative displacement and each successive coil in said single direction of relative displacement is wound in the opposite electrical sense to each adjacent coil.
The sensor of the invention of co-pending application Ser. No. 789,147 may be adapted to a multiplicity of circumstances and desired output signals, and, in a particular arrangement, said at least one secondary or sense winding has at least one coil having a pitch or extent in said single direction of relative displacement and said at least one conductive screen has a pitch or extent in said single direction of relative displacement which is substantially equal to the pitch or extent of said at least one coil, said at least one conductive screen having a dimension in a direction at right angles to said single direction of relative displacement which is substantially constant over said pitch or extent of said at least one conductive screen and said at least one secondary or sense winding having a dimension in a direction at right angles to said single direction of relative displacement which is substantially constant over said pitch or extent of said at least one coil.
Said co-pending application also discloses a sensing system comprising a sensor having a drive or exciting winding for establishing a forward or drive field, at least one secondary or sense winding in which a voltage may be induced in the presence of said forward or drive field, and at least one conductive screen within which eddy currents are generated in the presence of said forward or drive field to establish a counter-field opposing said forward or drive field, said at least one conductive screen and said at least one secondary or sense winding being displaceable relative to one another within said forward or drive field so that said at least one secondary or sense winding may be shaded by said at least one conductive screen to a varying extent to thereby vary the voltage induced in said at least one secondary or sense winding, said relative displacement of said at least one conductive screen and said at least one secondary or sense winding being limited to substantially a single direction of displacement, said drive or exciting winding being configured so that in the absence of said at least one conductive screen said forward or drive field is substantially even over said at least one secondary or sense winding in said single direction of relative displacement of said at least one conductive screen and said at least one secondary or sense winding, and said at least one secondary or sense winding being a two-terminal winding, and said at least one conductive screen having a surface region which is substantially parallel to a notional surface defined by said at least one secondary or sense winding, means for applying an alternating voltage to said drive or exciting winding, and means for processing the output signal from said at least one secondary or sense winding to provide a signal indicative of the relative disposition of said at least one conductive screen and said at least one secondary or sense winding.